This invention relates to the detection and correction of channels in error in a parallel multi-channel data handling system such as a magnetic tape storage system. More particularly, the invention relates to increasing the number of correctable channels in error among logically independent sets of channels recorded on the same medium without increasing the number of redundant channels per set.
In magnetic tape storage systems, industry standards have governed tape size, data format, and recording density. Conventionally, one-half inch width tape has been used for recording nine tracks thereon. By custom, data are recorded on 9 parallel channels with 9-bit bytes across the tape. Consideration in this invention is given to the recording and recovery of data recorded upon two or more logically independent sets of channels on the same medium.
Magnetic tape is soft and pliable. Unlike other forms of moving magnetic storage media, such as rigid magnetic disks, magnetic tape storage systems require the tape to move in non-uniform contact relation with one or more fixed heads as data is being transferred to or from the tape. Any loose particles of debris between head and tape or spots on tape with missing iron oxide causes loss of signal amplitude. Under the circumstances, it is possible for the read back clock in the erroneous channel to lose synchronization with the data in other, channels. Consequently, the recording and playback of data from the tape may be in error over very long segments. In this regard, the noisy length of tape poses a data recovery problem unlike that of either single shot noise or burst noise. Characteristically, single shot and burst noise induced errors are usually of finite duration. The reconstitution of the infected data is obtained by the use of error checking codes with cyclic properties. These codes are complex both in their theory and use. Illustrative of a magnetic tape storage device capable of variable density recording and utilizing a cyclic code for error detection and correction purposes is the IBM 3803 Controller for the IBM 3420 Models 4, 6, and 8 Tape Drives. The code actually employed is described in Hong et al U.S. Pat. No. 3,868,632.
The present commercial practice is to reserve two out of nine channels for recording redundant information about the remaining seven channels of the set. This enables up to two channels in error to be subsequently corrected. If there is contemplated the recording of two or more sets of parallel channels on a tape, there would continue to be reserved two redundant channels per set. With this format in mind, suppose one set of channels contained three channels in error while a neighbor set had none. It is clear that using present error checking techniques, there would be no effective way to take advantage of the unused redundancy of one set in order to assist in correcting multiple channels in error in another set.
The prior art is typified by Patel reissue U.S. Pat. No. Re. 28,923; Hong et al U.S. Pat. No. 3,868,632, and Louis IBM Technical Disclosure Bulletin, Vol. 14, page 3846, May 1972. In this regard, both Patel and Hong are directed to error correction in a nine-track tape. That is, where both the data and the redundant channel are part of the same logical set. Louis described a method of record recovery from a plurality of tapes by the calculation of a data value replacing unavailable data from a simple parity modulo 2 added to the remaining available data.
The limitation in error correction among multiple and logically independent sets of channels is to be distinguished from methods of correcting up to three known erroneous channels in a single set. The latter correction methods utilize codes which require an equivalent of three redundant channels as described by Patel in U.S. Pat. No. 3,851,306. To use such methods and their underlying codes requires three redundant channels in each of two interleaved nine-channel sets of an 18-channel tape system, that one could correct up to three known channels in error in each set. However, the redundancy of three channels per set of nine is wasteful, especially in view of the expectation that the occurrence of three erroneous channels in both sets simultaneously is highly unlikely.